There are numerous applications, such as hands-free mobile communication, telephony, and hands-free speech recognition, where acoustic signals are acquired by a far-field microphone. In such applications, the acquired acoustic signals often include a large amount of noise, e.g., traffic, crowd, radio, TV, wind, or other ambient noise in the environment. The noise interferes with understanding and otherwise processing the acquired acoustic signals, particularly, in speech recognition.
Doppler techniques have been used for a number of applications. U.S. Pat. No. 6,251,077 issued to Mo et al. on Jun. 26, 2001, “Method and apparatus for dynamic noise reduction for Doppler audio output,” describes a method suppressing background noise in spectral Doppler images using adaptive noise-reduction low-pass filters. U.S. Pat. No. 6,773,400 issued to Njemanze on Aug. 10, 2004, “Noninvasive transcranial Doppler ultrasound face and object recognition testing system,” describes a method for measuring a baseline blood flow velocity in cerebral arteries while simultaneously testing the subject with face and object recognition tasks. U.S. Pat. No. 6,773,403 issued to Kim et al. on Aug. 10, 2004, “Ultra-sonic apparatus and method for measuring the velocities of human tissues using the Doppler effects,” describes a method for measuring a velocity of tissue components by sampling echo signals reflected from the human body by generating a frequency distribution of data containing the velocity.
It is desired to use ultra-sonic Doppler techniques for enhancing acoustic signals.